Due to their extraordinarily good ratio of stiffness or strength to density, core composites have a broad range of application especially in the field of aircraft construction.
Well-known conventional core composites are generally formed of an upper and a lower cover layer or cover ply, between which is located, for example, a honeycomb-type core structure formed of vertically extending cells with a hexagonal cross section, for increasing the stiffness of the resulting composite sandwich structure.
For example metallic corrosion-protected aluminum foils, or non-metallic materials, such as Nomex®- or Kevlar®/N636-paper for example, are used for forming the core structure. Both the Nomex®-paper as well as the Kevlar®/N636-paper are coated with phenolic resin in a submersion process for increasing the mechanical strength thereof.
The provision of edge seals in such core structures, as represented by the above described true honeycomb structures, is not problematic. Due to the small-volume repeat units represented by the individual hexagonal cells extending perpendicularly between the cover layers, seal material for the formation of the edge seal can be troweled, painted, filled, pressed, foamed or poured locally into edge regions of the core structure. Thereby the seal material is confined in the cells directly in this edge region, and cannot flow or spread uncontrollably into the remaining interior of the core structure away from the edge region. The material forming the edge seal can, for example, be a curable synthetic plastic material, for example in the form of a synthetic resin and/or synthetic plastic foam.
In contrast to the above described core composites having true honeycomb cell configurations, in which a spatial limitation or bounding of the cells is always present, new types of core composites, especially formed of three-dimensional folded or pleated comb structures, comprise an open or drainable structure. Namely, such pleated or folded core structures include fold or pleat valleys that form open channels extending continuously in the plane of the composite structure, i.e. along or parallel to the cover layers from edge-to-edge of the composite structure. Thus, the core channels of such a core composite remain drainable or ventilatable through the edges even after the opposite major surfaces of the core structure have been covered with the cover layers. Thereby, for example, it is possible to guide various types of lines (e.g. electrical lines, hydraulic lines, water lines, air lines, etc.) through the core composite without previously having to cut or machine a passage therethrough while impairing the mechanical properties of the core composite.
If, for the formation of an edge seal of the above described drainable core structures, a pasty or viscous curable seal material is introduced into edge regions of the core structure, then this material can spread out, depending on the viscosity, more or less uncontrolledly throughout the open channels of the open core structure. In other words, the seal material would not remain confined to the edge region, but rather could flow or run from the edge region freely deeper toward the center of the core structure along the open fold valley channels. Thus, viscous or pasty substances can spread out uncontrollably over larger distances when using open drainable core structures. In contrast, in true honeycomb structures, basically only a limited number of structural units (e.g. hexagonal cells) are opened by a separating cut at the edge of the structure, so that any seal substances introduced into the open cells in the edge region are always spatially limited to those cells. Thus the standard edge-sealing methods according to the state of the art are not usable for providing spatially limited edge seals in open drainable core structures.